text:
Is there a framework for inter-country rules enforcement?
I have a US pilot's license and if I do anything foolish or infringe an important rule while flying in the US, the FAA can take action such as order more training, suspend my license for a while, or take it away forever.
I heard a heated pilot/ATC exchange between a New York region controller and the pilot of a European airline in which the pilot would not follow vectors.  (I have my own opinion about who was in the wrong.)
If the FAA wanted to 'punish' the European pilot or decided he was too dangerous to be allowed to fly over US, what would be the process and what is the most severe punishment the pilot could receive?  I presume there is an overarching framework the US and the EU have signed on to.

The general framework for international civil aviation is the Chicago Convention, made of the Convention and its annexes. Under the Convention, scheduled flights over a foreign territory requires a permission which can be given under related terms and conditions (art. 4).
In the US, foreign airlines operates in compliance with US laws (FAR part 129) and an individual agreement (OpSpecs) with the US administration.
This agreement can be used to enforce compliance, as stated at §12.3.1:

12.3.1 Suspension or Revocation of Operations Specifications (OpSpecs). The compliance and enforcement program provides a wide range of options for addressing noncompliance. In addition to referring apparent violations to foreign governments for appropriate handling, options include administrative action in the form of a warning notice or letter of correction, the suspension or revocation of OpSpecs, civil penalties, injunctions, and referrals for criminal prosecution.

This may take the form of referring apparent violations to foreign governments. As @ymb1 mentions in his answer, this is not very effective.
But there are other possibilities from fining the airline to suspending the authorization to do business on the US territory (suspension or revocation of OpSpecs).
For example, operating at San Diego (KSAN) is allowed in OpSpecs only after PIC get a special qualification for Rising terrain close to runway. Lufthansa, a major airline, operated off their OpSpecs, they were fined for non-compliance when PIC where caught regularly landing at San Diego:
FAA Proposes $6.4 million Civil Penalty Against Deutsche Lufthansa Airlines
Such action against an airline of any country is possible because:

The Convention reaffirms the sovereignty of a State for its own airspace (art. 1), and
The action is not discriminatory, it applies equally to airlines from any country (art. 44g).

Thanks for this additional information.  It expands usefully on ymb1's answer.


text:
How can I tell the difference between Spitfires and Hurricanes in photos?
The Spitfire was one of WW2's iconic aircraft, its fame coming from its spectacular performance in the Battle of Britain. But in that battle, Hawker Hurricanes outnumbered Spitfires 2:1.
Often articles about Spitfires use images of Hurricanes. How can I tell the difference between a Spitfire and Hurricane visually?

I'd dispute that claim about Spitfire. Zero is quite iconic too.

Let's just say it's "one of" WW2's most iconic aircraft. The P-51, Me109 and Zero are all pretty famous, along with plenty of others

I don't think you could dispute that it's Britain's most iconic WW2 aircraft, though. There are certainly other distinctive British aircraft from that period, but none so famous.

There are quite a lot of differences, some more subtle than others, but I tend to find this is the best angle for identification: the side view. Although as we'll see later in this answer, the top/bottom view has a fairly obvious difference, and there's a clue that we can use to identify from the front or rear

So what are you looking at?
On the left is the Hurricane. It's slightly larger than the Spitfire, although that's not always a great help from distance. Perhaps more usefully, the fuselage is fundamentally different shape:  I'll elaborate on this in several areas where we can directly compare.
Firstly, the rear fuselage. At first glance the two are very similar, but the Hurricane has a much more pronounced "drop" to the tail, while the Spitfire's is a "rise".
What do I mean by that? Follow the line of the top of the cockpit canopy: the Hurricane's fuselage behind the canopy drops away, while the bottom of the fuselage is relatively flat. On the Spitfire, it's reversed - the top of the fuselage follows the line of the canopy, while the bottom comes up to meet it. It's a subtle difference at first glance, but once you've seen it once it's a very clear distinction. When looking at the Hurricane's rear fuselage, it almost appears to have a "dog leg", where the rearmost area drops in relation to the middle of the aircraft. The Spitfire's follows a much more continuous line
The Spitfire is also clearly much more sleek - it's a "pencil" shaped fuselage, long and thin and curvy. The Hurricane is more blunt and "solid".
Other differences? Look at the tail. The Spitfire's is relatively small and curves upward from the top of the fuselage in a smooth curve, while the Hurricane's comes off at a sharper angle. Similarly the Hurricane's horizontal stabilizer is higher - almost in line with the top of the fuselage.
The Engine cowling shape, as with the cockpit, is different: where again the Spitfire's continues a nearly straight line from the bottom of the cockpit, the Hurricane's has a distinct curve.
If you can get closer, you will also note that the Hurricane's fuselage is almost "ridged" - it's a metal frame with a fabric covering, and that gives it a bumpy appearance, vaguely visible on the above photo.

What about if you can't get a side view, though? Here's the Spitfire (below) and Hurricane (above) from the front... but you can see the same from the rear
Well from the front, there's a single very nice clue under the fuselage: the radiator housing: that box-like structure hanging from the bottom of the plane. The Hurricane has one, and it's on the centre of the fuselage. The Spitfire's is not, it's under the right wing. On some later models, there are two, one under each wing.
The next photo is included mostly for the wing shape, but the aforementioned radiator boxes are visible too: from this, we can tell the Spitfire is a relatively late war model: a Mk IX (9) onward: although there are other subtle clues that suggest this is a much later model... the 5 bladed propeller indicates a Griffon engine, so a Mk XIV (14) or later. Anyway, we can see the Hurricane's single centre radiator, and the Spitfire with one under each wing

And finally, how about the bottom? Well, the Spitfire's wing is extremely distinctive: it has an "Elliptical" wing, shared with only one or two other aircraft. Once you can recognise the Elliptical wing, you can recognise the Spitfire very easily in flight. The Spitfire's wing, particularly the trailing edge, is very curved compared to most other contemporary aircraft. The Hurricane's wings are much more conventionally shaped, with both leading and trailing edges being straight (a trait shared with aircraft like the Typhoon/Tempest, P-51, Me109, A6M "Zero" and most other aircraft from the WW2 era)

(source: modelingmadness.com)
Late War Spitfires (the above is a Mk 22) are even more distinctive: they have a "bubble" canopy that protrudes entirely from the fuselage.

"Perhaps more usefully, the Hurricane has more straight lines, while the Spitfire's more curvy" I'm not sure that's useful at all. Looking at your first photo, the Spitfire has a straight line from the top of the windscreen to the start of the vertical stabilizer, from the bottom of the windscreen to the front of the nose and from the back of the wing to the bottom of the tail. The Hurricane is curved in all of those areas.

And all of these photos need credits.

Hmm, I thought adding the photos from a URL automatically added credits: there was something at the bottom of the post with the links! I'll look at adding them back in. The "Curves vs straight lines" thing is more of a "Let's discuss this" rather than a hard and fast rule: I then point out several areas further down the post where we can apply it to specific areas like the wings and rear fuselage sweep

The Spitfire's elliptical wing has always been the dead giveaway for me. I honestly cannot think of another WWII-era aircraft with that planform, certainly nothing as well-known as the Spitfire.

There are several others - the He112 (competitor to the Me109), He111 bomber, and the A5M (the predecessor to the famous "Zero") all used the elliptical wing. Only the He112 and A5M are fighters, though, and the He112 was limited to a few prototypes, while the A5M is unlikely to be confused with the Spitfire.

Oh, and I'm missing the most obvious example... the P-47 Thunderbolt https://alexestep.files.wordpress.com/2011/11/estep_p47_topdown.png Although it's significantly bigger than a Spitfire/Hurricane, and the radial engine is a dead giveaway

@JonStory, unfortunately no, the photos still get copied. Besides, you should make them links to the original page that contains them and that can't be done automatically.

I joined this community just to upvote this.

I've read through this 5 or 6 times since it's been posted, and I want to up vote every time. Interestingly, I just noticed that the Mk 22 Spit has lost the nice, gentle leading edge curve into the vstab that made the earlier ones easy to identify. It's got a much sharper leading curve now and the leading edge is much more convex than the nearly concave leading edge in the 1st image.

@FreeMan yeah that's a feature of the Mk 22/24 Spitfires and the Spiteful (based on the Spitfire), and was part of a new larger tail which was needed because of the requirement to balance the torque of the Griffon engine and the much stronger slipstream from the propeller. The Spiteful also had a completely different, straighter wing more akin to the P-51 etc, while the Mk 22/24 wing had a kind of hybrid elliptical wing

I built models of each of these aircraft as a kid. And also saw them repeatedly in the Commando comics I read. From the standpoint of a child who knew very little about aviation but had built them as 1/32 models, there are two standout differences:
The Spitfire has a distinctive elliptical wing. The Hurricane has a more regular wing that is rounded.

The Hurricane has a rectangular canopy. The Spitfire has a rounded canopy.
Admittedly these general terms are not the best to describe the canopy, but there is a distinctive visual difference in the canopy. Wikipedia states the Hurricane has a distinctive "hump-backed" silhouette.

Both are very cool. But the Spitfire is somewhat cooler.
Even a 9 year old knows that.
Wikipedia has a good image of the Battle of Britain Memorial Flight. The Spitfire wing should be obvious, and one of the Spitfires has the clipped wing. Two of the Spitfires also have invasion stripes.

And some extra goodies.

The canopy isn't a great way to identify Hurricane and Spitfire: Early mark Spitfires had a conventional (rectangular) canopy, and there are Hurricane examples with the Malcolm (rounded) hood. And then there are Spitfires examples with neither.

If you believe the canopies to be very similar, please find an example.

Here's a spitfire canopy without so much as a curve in sight... http://spitfirespares.co.uk/Website%20Products%20427/spitfire.jpg and another here http://www.raf-in-combat.com/wpsite/wp-content/uploads/edd/2014/11/tn_Spitfire-08.jpg - Early ware Spitfires did not have Malcolm hoods nor Bubble Canopies, and cannot be distinguished from Hurricane's based on "Rectangular vs Curved canopy". Few survived the war, but if identifying Spitfires in wartime photos/films, the canopy isn't a great indicator. Here's another http://www.warfaremagazine.co.uk/assets/images/articles/medium/20120413122002.jpg

You should mention other things like inward and outward facing gear and the intake.

All of these pictures need to be credited.

This is a Spitfire from its most distinctive viewpoint. Note the elliptical wing. Some later models had the tips of the wing clipped.

This is a Hurricane.

This problem is complicated by the fact that there were many variants of the Spitfire (especially) and the Hurricane. Here are some easy-to-spot differences.
This is a Spitfire again.

Note the Spitfire has  bubble canopy. Hurricane has a razorback.
The Spitfire has a rounded vertical stabilizer. The Hurricane is rounded.
Most Spitfires have extended barrels for cannons. Most Hurricanes do not have cannon (some models did).
The hurricane was fabric covered. Note the fabric ripples on the tail.

The elliptical wing works, but there are plenty of Spitfires without bubble canopies. Equally there are Spitfires with 0, 2, or 4 cannon, and Hurricanes with 0, 2, or 4. You're comparing a very early Hurricane to a very late Spitfire: for comparing similar age (Mk I, II, V) early war aircraft, this really doesn't apply

Even the earlier Spitfires have a bubble canopy of sorts.
https://upload.wikimedia.org/wikipedia/commons/0/00/Spitfire_IIA_P7350.jpg Even on the early Spits, the canopy distinguishes it from a Hurricane.
The A and D wing Spits do not have cannon tubes but most were B or C wings. Likewise, most Hurricanes were were machine gun armed (as noted above). If you see a cannot tube it is most likely a Spitfire.

There are Hurricane's with Malcolm hoods, and plenty of Spitfires with conventional canopies. You give three "likely" identifications but two can be shared with the Hurricane and therefore aren't truly ways to identify the aircraft. Only the Bubble Canopy is a distinctive feature, and isn't enough to distinguish all Hurricanes from all Spitfires

All of these pictures need credits.

@JonStory Not all Spitfires have elliptical wings.

Yes they do. They don't have identical elliptical wings, but they have a variant of the original wing design. Either with clipped/extended wing tips, or the Mk 22/24 "adjusted" elliptical design. It's still an ellipse, however, and clearly distinct from the Hurricane's. The "Spitfire without elliptical wings" isn't a Spitfire, it's a Supermarine Spiteful.

@JonStory The Mk XII has clipped wings. It's still based on the elliptical shape, but I wouldn't expect a layperson to be able to identify it reliably.

@JonStory Can you provide a reference for the Malcom-hood-equipped Hurricane? I've googled, and all that I can find are modellers' what-if designs.

I don't believe there are any surviving photographs, but I'll try to dig out the reference regarding the test examples if I can find it.

Certainly on many of the earlier model spitfires, the aerial was closer to the cockpit canopy than the hurricanes. Later models had much finer (thinner) aerials, more difficult to see.

Hello and welcome to aviation.se. The answer in it's current state is quite short and may be better suited as an comment. Edit your answer to include relevant data to back up your statement such as pictures or references.

Welcome to aviation.SE!  To add to @jklingler’s comment, it may also be helpful to add pictures to show the differences you mention.

The main gear in a Hurricane retracts inwards (wheels together), while in a Spitfire it retracts outwards (wheels apart)

You answer is quite confuse or not elaborate enough. What do you mean by closing a leg?

I think it means that the Hurricane's gear retracts inwards, the Spitfire's retracts outwards. While accurate, it's only helpful if you have a good view of the gear, and doesn't help for most of the pictures in the current answers. Without more detail it seems it would be better as a comment.

When both Hurricanes and Spitfires were a common sight over England's skies, there was a saying: greenhouse = Hurricane, blister = Spitfire. This refers to the cockpits. The Hurricane has a many-paned canopy whose top is flat and level with the top of the fuselage. The Spitfire has a curved canopy which sticks out from the top of the fuselage.
As others have pointed out, there were many variants of both aircraft, and some variants lack the biggest recognition features. One variant of the Spit even lacks the distinctive elliptical wings, with cut-off straight wing-tips instead. Although I don't have the exact figures, I think the cockpit canopy is the most reliable indicator.
You can't see in the photos people have posted, but the sound of the Merlin engine which most Spitfires had is very distinctive as well. If you've heard it once, you'll recognise it again, and you won't confuse it with another aircraft.

One joke I've heard from today's aviation enthusiasts is that a Spitfire looks very similar to a Seafire, while a Hurricane looks almost the same as a Sea Hurricane. The joke is of course that those two are both naval variants, so obviously they look very similar to their land counterparts, but if you can't tell them apart to start with, that doesn't help.

The cockpit canopy isn't the most reliable indicator - as discusssed elsewhere in this thread, there are examples of both Hurricanes with the Malcolm hood, and Spitfires with a conventional ("Greenhouse" canopy)

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text:
What is the small cylinder on the firewall to the left of the battery?
What is this and should the yellow wire be soldered to it?
Any ideas on preventing the wire from detaching again?

I checked the C150 parts manual but didn't see it. Is there somewhere else I should be looking, or did I just miss it?

Are you working on this airplane yourself? If so you need to be aware of your local regulations.

Whats under door #1? that looks like it could be a cable connection cap, could be an antenna connector. Showing us whats under the cap could be of big help.

@GdD: I'm not modifying anything without supervision from an A&P, I'm just curious about what goes on with my plane and prefer to post here where the answers can potentially help others.

@Zaz: Freehand red circle might be usefull to save time getting what you mean. :P

That is a fuseholder for the optional clock or optional Hobbs meter. The yellow wire probably should be connected to it but you should have a certified mechanic inspect it.

Interesting to see the free wire maybe the one directly and permanently connected to the battery anode.

Thank you! Where did you find this? I couldn't find it in the 1965 POH or parts manual.

I recognised it as a fuseholder and had a hunch it would be for a Hobbs meter. I then did a Google search and found multiple references.

Sorry to bother you, but I have a couple of questions: My plane has a wind-up clock and no Hobbs; also, there's no voltage between the wire and the cathode or anode. What does this mean? Could it be the generator fuse that is no longer used now the plane has an STC alternator?

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Are 100 hour inspections required to rent out a plane?
According to 14 CFR § 91.409 - Inspections:

(b) Except as provided in paragraph (c) of this section, no person may operate an aircraft carrying any person (other than a crewmember) for hire, and no person may give flight instruction for hire in an aircraft which that person provides, unless within the preceding 100 hours of time in service the aircraft has received an annual or 100-hour inspection ...

It seems to me that:

By renting out my aircraft, I am not operating it for hire.
The aircraft is not carrying any person other than a crewmember.

Am I wrong about both of these, or is it legal to rent out aircraft without 100hr inspections?

Don't forget some Commercial insurance - unless you are adding each renter as named pilot on your policy and the pilot meets the minimum requirements for your insurance?

@CrossRoads: Couldn't I just request they have renter's insurance?

I don't know. I suggest you call your insurance provider and discuss.

Renter's insurance will protect the renter, not you. It may pay for damage the renter caused to the plane and any liability on their part, but it won't protect you from a liability claim. The renter or their estate or a third party may come after you if the accident is caused by poor maintenance or other cause attributable to you. You need insurance to cover you when renting your plane. Talk to you insurer.

The FAA provides an answer in this Letter Of Interpretation. In this case they discuss it in the context of a company owned aircraft but they tend to care more about how its operated than who owns it.

For instance, if Fly By Knight is going to operate the aircraft to
provide flight instruction for hire, then the aircraft must have had
an annual or 100-hour maintenance inspection within the preceding 100
hours of time in service. If Fly By Knight is going to rent the
aircraft to a customer, however, and does not provide the  pilot, that
aircraft need not have an annual or 100-hour maintenance inspection
within the preceding 100 hours of time in service. See Legal
Interpretation, Letter to Berry Rackers from Joseph Brennan, Associate
Regional Counsel (May 3, 1984).


text:
How long can an average aerobatic plane sustain 0g?
How long can an average aerobatic plane sustain 0g safely?

for 0g the plane has to be in freefall, so unscientifically speaking it would be the difference between max achievable vertical speed in a climb and max safe vertical speed in a dive, divided by 9.8 (for speeds in m/s). A "vomit comet" can sustain it for about 25 secs, so I'd imagine an aerobatic plane would be similar, or perhaps less because it's slower.

There's also the consideration of lubrication flow through the engine.  While oil delivery and recovery systems are pressurized, lubricant travel through an engine/bearing/etc. is typically unpressurized, letting fluid stagnate in hot spaces.  I'd think this would lead to some temperature rise (how high?  not sure) in those spaces at the least.

The longest possible 0g experience is on a parabolic Flight http://www.asc-csa.gc.ca/eng/sciences/parabolic.asp.
As Devil07s great answer explains the time a plane can fly "at 0g" depends on its vertical speed at start of the 0g time.
Lets take a typical air acrobatic plane. Its maximum speed is 220knots ~ 400km/h.
The plane looses some speed before entering the flight pattern (until it reaches 45° pitch) and gains some more speed before reaching 0° pitch after leaving the pattern, but lets ignore that for the ease of the calculation.
At the moment the plane enters the  parabolic Flight pattern with 45° pitch its vertical speed is
IAS/sqrt(2) = 400km/h*1.414 = 283km/h = 79m/s
Choosing a higher pitch will create a higher entry speed by calculation but the entry speed will also be reduced by the longer "rotation time" and structural limits of the plane.
Now it follows the parabolic Flight pattern until it reaches the culmination point (0km/h vertical speed):
time = speed/acceleration = 79m/s / 9,81m/s² = 7,9s
The plane needs exactly the same time to reach the "exit speed" of 400km/h. Therefor the total time an Extra EA-300 can fly in 0g is approximately:
15 seconds.

An aerobatic aircraft (as well as any aircraft) can sustain 0g until the pilot pulls up, the aircraft hits the ground, or reaches terminal velocity.
In order to sustain 0g  the aircraft must be accelerating towards earth at $9.8 m/s^2$ (meters per second squared). That means there isn't much time before the aircraft reaches its maximum speed. Once the aircraft can no longer accelerate at the same rate of gravity, then the occupants will no longer feel 0g. In sky diving (and physics) this is called terminal velocity, which is the velocity at which air friction prevents the object from falling any faster.
Acceleration is change in speed, so naturally you don't have to be going straight down to be accelerating towards the earth (we're talking about vertical speed). If you are going up at 200kts and begin to decelerate at exactly $9.8m/s^2$ you will "feel" like you are in 0g (free fall) even though you are continuing to go up for a few seconds. But this will only last for a few seconds before the aircraft momentarily reaches its apex and you and the aircraft begin moving towards the earth. If the aircraft can match exactly the acceleration of gravity, then you will continue to feel weightless (i.e. 0g).
But you can't continue to accelerate indefinitely towards earth without either reaching terminal velocity or exceeding the aircraft's maximum speed. So the limit of the sustained 0g flight is the pilot pulling back on the yoke and climbing to prevent the destruction of the aircraft and death of all on board.
However, even if the wings broke off of the airplane, and you were plummeting towards the ground in the fuselage of the aircraft, you would only remain weightless so long as the fuselage continued to accelerate towards the earth at $9.8m/s^2$. At some point, if you had enough altitude, the drag (wind friction) on the fuselage would prevent it from continually accelerating towards earth and you would reach an equilibrium speed where the force of gravity equals the force of drag and you would no longer feel 0gs.

Excellent answer!

Thanks for the great answer! Since the OP asked how long it would be awesome to add a calculation of the 0g-time based on the initial speed of the airplane.

Theoretically, a plane in LEO could be in 0g for a few years.

An average aerobatic plane, with 300 HP 540cubic inch engine (IO540) could not fly to LEO (low earth orbit) tho.

@CrossRoads True, but Elon Musk could put it in LEO, just for the fun of it.

That depends on a whole heap of things. Zero g is experienced during a ballistic trajectory. Wikipedia lists as time spent during ballistic flight:
$$ t = \frac {2 \cdot V \cdot sin\theta}{g} \tag{1}$$ with V = starting velocity in [m/s] and $\theta$ the starting angle. The equation is valid for vacuum, which would be equal to our situation where thrust continuously equals aerodynamic drag.

For any velocity, if we want to maximise time spent we need to start at 90 deg, as this plot shows: time in seconds as function of starting angle at starting speed of 100 m/s. But that creates a bit of a practical problem at the apogee, the aircraft will fall back tail first, do a hammerhead, and we're not in zero g anymore. So let's take an angle fairly close to the maximum where the pilot could maintain parabolic flight using his incredible skill: 60 degrees.
 Image source
We're in an aerobatic aircraft, which is a very wide range actually: jet fighters are fully aerobatic as well. If we take a typical Red Bull Air Racer as one delimiter and an F16 as the other one, we have a speed range of between 100 and 600 m/s. For a ballistic flight path starting at 60 degrees, we get the following flight time as a function of starting speed. Between roughly 18 and 106 seconds.

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text:
stall on commercial airliner - why is there no better on-board software?
STALL:
I believe a simple software update to the existing on-board flight computers could - possibly - save lives.
Pilots need to be indicated possible errors by the on-board computers more detailed and more clearly.
Backround: after having watched these 3 reports on:
       1) crash Air France flight 447 to Rio into the Atlantik in 2009,
          an A330-203 (reason:stall, technical problem: air speed sensor(s)
          frozen)

   2) crash of Air New Zealand owned XL-Airways A320-200 in 2008
          (reason: stall, technical problem: pitch sensor frozen)

       3) crash Birgenair 301 in 1996 of 757-200 from the Dominican Republic
          (reason: stall, technical problem: air speed sensor)

In general, these very often occuring problems with a faulty or temporarily faulty sensors (i.e. frozen) such as an
air speed sensor should be directly indicated to the pilots by better software in the current on-baord computers.
A more relevant message to the pilots could and should be indicated.
For example: "POSSIBLE ERROR - Air speed sensor. Check pitch.  If pitch within the range of xx - xx then possibly no stall situation present".
[the pitch value to be used could come from an internal look-up table in the on-board computers memory for the given aircraft under its current load conditions].
In the case of Air France, had the pilots taken no action other than maintaining altidude and air speed, nothing would have happened.
As they took incorrect action, assuming there was a stall situation, the whole problem emerged in the first place.
The on-board computers should "know":
IF the air speed does not match +/- the air speed the plane should be doing for its given amount of thrust by the engines
[lookup-table in software],
then: (assuming engines and plane is ok), there must either be a problem with the pitch sensor(s) or air speed sensor(s).
As these sensors are doubled, they are checked against each other.
If , for example, two air speed sensors differ from each other, as I believe it is currently done, then it is
assumed by the on-boad computers that one of these is faulty, but the on board computer does not conclude which one it may be and thus does not "know" what air speed it should use.
BUT, as explained above, WHY NOT: when checking against prevailing thrust and pitch and other relevant sensors, the on-board
computers can come to an estimation for the given aircraft's actual air speed currently present [lookup-table in on-baord computers memory].
And then, this information with the big mention of POSSIBILITY should be indicated in the warning messages,
to give pilots a head start in analyzing and identifying the problem.
Currently, it seems to me, that unneccessarily, much valuable time is spent by the piolots having to first analyze the
situation from scratch and then decide on which action is needed  to regain control from the stall, by which time it is often too
late as  altidude/air speed are by then too low for a recovery.
This was the case for (1) Air France as well as for (2) XL-Airlines.
Also, (1, Air France) had the problem that if one pilot pulled the stick backwards and the other pilot moved the stick forward,
the action was cancelled out and no indication was given as far as I know, so pilots lost further valuable time arguing.
In (2, XL-Airlines) also had the problem that the A320-200 would in the stall emergency situation only respond to the
trim wheel and not to the stick.
The pilots realised this too late.
The on-board computer indicated this on its screen, but it was overlooked, as obviously very unusual.
It is as if when driving a car, suddenly it is no longer the steering whel we need to change direction but the handbrake.
What were the developers thinking here, I really do not understand.
Last but not least: the constant alarm noise should after a while be tuned down somewhat and the sound level reduced - it does nothing to get the
situation resolved faster but instead causes stress levels to soar even higher, which is detrimental to pilots acting
calmly and in getting the situation under control..

multiple sensors can fail at the same time, computer programs can have bugs

The on-board computers should "know": IF the air speed does not match +/- the air speed the plane should be doing for its given amount of thrust by the engines [lookup-table in software], then: (assuming engines and plane is ok), there must either be a problem with the pitch sensor(s) or air speed sensor(s) there is quite a lot more stuff that can make the lookup table not match the measured speed, including software bug, engine malfunctions, etc.

"Last but not least: the constant alarm noise should after a while be tuned down somewhat and the sound level reduced" - this sounds like a good idea, but note the suppression of stall warnings actually contributed to the AF447 incident. (Stall warnings did not sound at very high angles to prevent false warnings, apparently.)

I'm not a pilot, but a software developer. You can make (and often should) software more clever. But making software more complex is also dangerous! You really should think twice before adding any kind of unnecessary logic. Instead of given any suggesstions and assumptions, I would prefer to tell the pilots than sensor x return unusual values or differs from sensor z and y if present. Basically our rule is KISS, KEEP IT SMALL AND SIMPLE.

Sorry, @Peter, KISS originated as an acronym in the context of Contract Bridge in the early 50's, and it stands for Keep It Simple, Stupid. (Thus it is not KISAS.)  (LOL -- which acronym with a completely different meaning than the modern internet meaning also came for contract bridge, around the same time)

In the case of AF447, the aircraft was capable of flying safely, and should have been flown safely.  No software would have saved the poor souls on that flight. What were the developers thinking here - many times more than you or me.

@Simon "No software would have saved the poor souls on that flight." Unless the software gave remote control of the aircraft to a competent pilot on the ground.

You are right, the computer should had come to a reasonable estimation of the speed, even in the presence of false airspeed data. The computer between the ears of the captain, that is. What does it help to try to cover every eventuality in software when you have badly educated bus drivers in charge? Sadly, the pay and treatment of pilots by their management makes it obvious they are seen as bus drivers, and we should not be surprised if they behave like this sometimes. I am more amazed when they rise above that, and they do that in 99.9% of all cases.

For example: "POSSIBLE ERROR - Air speed sensor. Check pitch. If pitch within the range of xx - xx then possibly no stall situation present". [the pitch value to be used could come from an internal look-up table in the on-board computers memory for the given aircraft under its current load conditions].

An aircraft can stall at any pitch angle and many different speeds, so pitch is not a good indicator of the onset of a stall. A better indicator is Angle of Attack, but in many cases those sensors freeze over. You cannot measure AoA without a physical sensor using MEMS or INS components.

The on-board computers should "know": IF the air speed does not match +/- the air speed the plane should be doing for its given amount of thrust by the engines [lookup-table in software], then: (assuming engines and plane is ok), there must either be a problem with the pitch sensor(s) or air speed sensor(s).

A problem with the pitch sensor/airspeed sensor does not automatically mean a stall, and also should not present additional stall warnings without the actual stall condition. How would you separate the two? Just like you don't want them to overlook a warning, you also don't want to present too many that aren't relevant as it will increase the time it takes to figure out what is going on.

BUT, as explained above, WHY NOT: when checking against prevailing thrust and pitch and other relevant sensors, the on-board computers can come to an estimation for the given aircraft's actual air speed currently present [lookup-table in on-baord computers memory].

Imagine flying from clean air into the jet stream as the sensors fail. You haven't changed the thrust levels and suddenly you are presented with a tail wind which greatly reduces your airspeed. Engine thrust does not immediately react to the surrounding air mass, you can't say that for every condition Thrust X = Airspeed Y, its not a simple correlation especially given changing conditions.

And then, this information with the big mention of POSSIBILITY should be indicated in the warning messages, to give pilots a head start in analyzing and identifying the problem.

No, this is absolutely not a good thing to do. Like I said above, you don't want to present warning conditions about the possibility of a situation. The pilot should be able to infer from airspeed invalid warnings that the possibility of a stall is there, and fly the aircraft appropriately. Presenting too many warnings is worse than not enough.

Currently, it seems to me, that unneccessarily, much valuable time is spent by the piolots having to first analyze the situation from scratch and then decide on which action is needed to regain control from the stall, by which time it is often too late as altidude/air speed are by then too low for a recovery

Pilots are trained for stall recognition and recovery and the recovery procedure after recognition should be a reaction without a lot of thought. The problem with the Air France flight is that the pilots did not recognize the stall even after the airspeed sensors started working again. By the time the captain realized that one person was climbing while the other person was descending, it was too late:

02:13:40 (Robert) Remonte... remonte... remonte... remonte...
Climb... climb... climb... climb...
  02:13:42 (Captain) Non, non, non... Ne remonte pas... non, non.
No, no, no... Don't climb... no, no.
  02:13:43 (Robert) Alors descends... Alors, donne-moi les commandes... À moi les commandes!
Descend, then... Give me the controls... Give me the controls!
  02:14:23 (Robert) Putain, on va taper... C'est pas vrai!
Damn it, we're going to crash... This can't be happening!
  02:14:25 (Bonin) Mais qu'est-ce que se passe?
But what's happening?
  02:14:27 (Captain) 10 degrès d'assiette...
Ten degrees of pitch...

Exactly 1.4 seconds later, the cockpit voice recorder stops. That means when the pilot realized that they had the wrong control inputs at 2:13:42 it was less than 60 seconds from impacting the water, no way they would have been able to recover. So the take-away from AF447 was that pilots needed to be trained to recognize a stall and the proper recovery techniques. The pilots had regained control at 2:10:38 and the airspeed sensors were reading normally at that time. By 2:10:55 all avionics were functional. They put the aircraft into another stall at 2:11:03 when they tried to climb at TOGA power around 37,500 feet.
Again, the issue is overload. You don't want to overload the pilots with too much information. A stall horn and the red X over the airspeed indication should be enough for the pilot to realize to lower the nose and increase speed. AF447 was a wake-up call to commercial operators about the lack of hand-flying skills in todays pilots and quite a few good changes came from it including stall awareness training and recovery training on a reoccurring basis.

To extend on your good answer... "[pilots] needed to be trained to recognize a stall and the proper recovery technique" may be the core of the problem, as pointed out in this interesting reading of the accident by Vanity Fair: The Human Factor. Automation may actually contributes to this lack of pilot skills, "a better software" as the OP suggests could actually lead to removing the pilots from the cockpit, but that won't remove all the failures. I'm not sure this is what I want.

Thanks @mins, I completely agree. The erosion of hand-flying skills in modern airliner pilots is downright frightening. Usually an airliner flight is wheels-up-autopilot-on all the way to touchdown. I've heard that this is even a matter of company policy in some instances. The other sad part is that many of the pilots aren't paid enough to have GA as a hobby so they can't get into aircraft that require good old stick-and-rudder skills.

@mins Loss of flying skills is certainly a factor in accidents but I don't think its any more of an issue in FBW ac than a typical AP. They both pretty much require the same amount of hand flying. The truth is, on AF447, no matter how much analysis has been done there's just no clear answer explaining that pilots actions. Failing to recognize and recover from the stall is clear but how they got into the stall in the first place is another issue.

@RonBeyer My personal, non-pilot opinion (aka pure armchair speculation) is that the complexity of the Airbus FBW system throws pilots curveballs. When there's a malfunction and the control law changes, the pilot is handed an aircraft that doesn't fly the way they are used to. It's like trying to ride one of those old-time bicycles with the big front wheel. You'd eventually figure out how to do it but you'll probably fall a few times before you get the hang of it.

@TomMcW Again, its an erosion of hand-flying skills. Alternate law just means that the airplane does what you ask, you shouldn't be a PIC without knowing how to fly an airplane that responds like every other non FBW airplane. The issue wasn't really being in alternate law, it wasn't knowing how to fly the thing by hand without a computer in between. You shouldn't be learning how to fly a plane when you are responsible for 200+ lives, you should have hundreds of (recent) hours doing so already.


text:
How do the Kaman K-225 rotor "planes" work?
I had the opportunity to go to one of the Smithsonian Air and Space museums today and they had a Kaman K-225 on display. For those who don't know, the K-225 is a dual side-by-side helicopter design with an open cockpit:

Source: Own Work
The blades look to be very basic wooden design without any kind of variable pitch on the rotor head. What I found very odd were the "planes" on the rotors:

Source: Own Work
Each rotor seems to have one of these "planes" on it located about 2/3rds of the way from the root. Here is a close look:

Source: Own Work
I wish I could have gotten a better picture but it looks like there is a rod going from the main blade to the smaller "wing" on the device. The rotors seem to be solid, so I'm wondering...
What are the purpose of these "planes" and how do they work (I'm assuming to change the AoA of the blade)?

Sorry for asking this, but is "dual side by side helicopter" the correct definition? 'cause it sounds like it is two helicopters connected together, not simply an heli with two rotors O_o

Yeah, a "synchropter" or a helicopter with intermeshing rotors would be a better descriptor (https://en.wikipedia.org/wiki/Intermeshing_rotors).

These are a form of servo flap, commonly found on Kaman designs, but also on at least one early helicopter (namely, a coaxial rotor aircraft built by d'Ascanio (shown below):

As opposed to controlling blade pitch from the blade root using pitch links (which are, essentially, pushrods), as in a typical rotor head, the servo flaps allow for pitch to be controlled by a control surface on the blade (similar to how controls work on a fixed-wing aircraft).  The yellow "wing" is the flap, and the black item, I'd imagine, contains a hinge and actuator rod to pitch the flap up and down, generating a force at a small moment arm behind the blade, which then alter's the blade's pitch as desired.  Instead of using an actuator to twist the blade, you're using aerodynamics to do a lot of the work for you, similar to how a fixed-wing aircraft works (except, instead of changing the rotor's effective angle of attack, you're twisting it instead).
Hence, if you look closely at the first picture you posted, you'll noticed that there is no swashplate, no pitch links, and really nothing reaching up the rotor mast from the helicopter except for a structural brace.  This system does away with pitch links and their associated actuator systems and can decrease total system weight.  In addition, as the flaps are significantly more streamlined than a typical pitch link, there is a possibility for drag savings as well.  Compare the following two rotors heads -- the first is a semi-rigid 2-bladed rotor from a Bell 230, and the latter is a typical Kaman rotor head.  The Kaman head is a picture of simplicity when you realize the only bits sticking out in the wind, beside the blade grips, are droop stops.

Bell 230

Kaman H-43
However, there's no free lunch.  The price you pay for all this is that, instead of an actuator somewhere relatively "easy" (on the body of the helicopter), you have to put an actuator (albeit a much smaller one than would be required to drive traditional pitch links) out on the rotor blade and feed both control and power out to it.  Control and power isn't too bad -- it's already done for anti-icing gear to some extent -- but the actuator can be tricky, particularly, I would think, in terms of tuning blade dynamics.  Kaman (who built the K-225 and the highly successful KMAX, which also utilizes servo flaps) found a way to do this that works great, but it hasn't really made a splash in any other manufacturer's helicopters (to my knowledge).  Rumor has it that Sikorsky and Northrop put together an entry for DARPA's Unmanned Combat Armed Rotorcraft program that had servo flaps on it, but that program got cut in 2004...so who knows?  In the interim, swashplates have been the control method of choice for upwards of 80 years and work well for those who don't have experience with flaps.
Sources: https://www.helis.com/howflies/servo.php and rotor dynamics class.

I believe you are correct! Correction made.  Thanks!

Kaman have a swashplate and blade pitch control links on their helicopters just like any other. The blade pitch control links are inside the hollow rotor shaft - the swashplates are inside the helicopter at the bottom of the hollow rotor shafts.
Also instead of directly controlling blade pitch by using the pitch control links to twist the root of the blades like other helicopters each blade pitch control link connects to a small torque shaft inside each blade which drives those small flaps located near the end of blades (servo flaps) the small flaps motion twists the flexible helicopter blades to change their blade pitch.
This servo flap technique requires less force to control pitch than other helicopters. Most other helicopters require powerful hydraulic servos to drive the swash plate position - on the Kaman the servo flaps amplify the control force so they don't need much force to control the swash plate.
I noticed some people think the Kaman dual intermeshing blade helicopter has no swashplate or control links to the rotor blades - The reason there is a misunderstanding that it has no swashplate or blade pitch control links is because the swashplate is not located near the top of the rotorshaft like traditional helicopters where it is easy to see and the pitch control links are hidden inside the hollow rotor shaft. Their understanding of how this helicopter works is deficient but unfortunately this incorrect assumption is passed about.
https://patents.google.com/patent/US3217809
The Kaman patent to which I link shows diagrams and describes the swashplate control and shows the hollow rotor shaft and so on - the patent shows only a single rotor helicopter but the dual intermeshing design uses the same hollow shaft etc. The control linkage mechanism which controls the swash plate or plates is called the azimuth mechanism.
The Kaman design is extremely robust and low maintenance and the latest Kaman K-MAX twin inter meshing design is popular for logging and fire fighting among other applications. The dual rotor design has outstanding fuel efficiency, outstanding payload capacity and makes less noise as well.

Something you may want to know: Kaman called their version of the swashplate the azimuth on the Sea Sprite.  (The accident where the azimuth came apart and led to the two bolts being added to it happened at our squadron). To say that the servo flaps "control the swashplate" sound off to me; you might want to rephrase that.  As to not needing hydraulics to fly them (example, the Sea Sprite); the tail was a bit harder to control, but the aircraft was very flyable boost off. Have not flown the Kmax, can't comment on that one.  (Wasn't my down vote)

The swash plate controls the servo flaps which in turn controls the blade pitch by twisting the blade - i never said the servo flaps control the swash plate. !

note: It's Kaman, not kamen, named for the company's founder, Charles Kaman.


text:
How do I know if an arrival procedure (STAR) clearance includes altitudes?
Air traffic control (ATC) can issue a standard terminal arrival route (STAR) to aircraft entering the terminal environment. The STAR usually communicates a predefined route of flight (fix1, fix2, fix3, etc.). The STAR procedure usually includes altitudes too.
What will I hear from a controller when I am to follow the vertical guidance on the STAR vs. waiting for assigned altitudes?

In the U.S., the phrase "Descend via the (arrival)" means that you are cleared to the altitudes as published.  Without that phrase, simply cleared for the ___ arrival, then your clearance is the fixes, but ATC will assign your altitude explicitly. In this case, the published altitudes may later be assigned, so descent planning should still be based on making the next fix at its altitude. You just don't go below your assigned altitude until cleared lower.
FAA Explanations:
Starting on page 8 for "descend via" clearances: http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs400/afs470/pbn/media/rnav1_rnp1_rnav2/climb_descend_via_faq.pdf
More succinctly: https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/info/all_infos/media/2014/InFO14003.pdf

ICAO phraseology has the same phraseology for being cleared for arrivals:
.724  Freigaben auf Einflugstrecken
Clearances on a STAR

G: DESCEND VIA (designator) (level)
G: DESCEND VIA (designator) (level) CANCEL LEVEL /
   SPEED RESTRICTION(S) *AT (significant point)*

The phraseology for transitions is different from this:
   RNAV phraseologies

G: CLEARED (designator) TRANSITION
G: CLEARED (designator) TRANSITION AND PROFILE

The interesting part about STARs however is: Even when cleared to descend to a flight level or altitude after previously being assigned a STAR, the altitude restrictions of the STAR are in effect until cancelled by the air traffic control unit, as referenced above, e.g.:
The waypoint EXMPL has a lower restriction (minimum restrictions of FL100) and you are cleared to descend via the STAR to FL70, the fix still needs to crossed at or above FL100, unless that restriction is cancelled with above phraseology.


text:
Does lift-to-drag vary with anything beyond geometry?
The basic equations for both lift and drag show dependency on $v^2$, air density and some constants. At a glance, Lift to Drag ratio should depend on nothing else than plane geometry. Although I realize in aerodynamics things are rarely that simple - and so, I wonder how constant these constants actually are?
In particular - let's say we use a very efficient air-augmented rocket engine (so thrust doesn't degrade with air pressure) of a moderate thrust, sufficient to achieve take-off and a modest climb rate. We also somehow manage to shield the airplane from overheating. As it climbs, air density drops, speed rises - could it reach orbital speed eventually? Or will it encounter conditions where it can neither accelerate to gain more lift, nor climb to reduce drag?

Yes, it is entirely possible that the airplane will hit a wall where no more acceleration is possible while climbing. This is called the sound barrier. Besides the geometry, both the Reynolds number and the Mach number influence L/D.
As a rule of thumb, the zero-lift drag coefficient at least doubles when accelerating to Mach 1. Slenderness and sweep help to keep this rise down, but some is inevitable. Below is a plot with test data from the linked NACA Research Memorandum A55F06 for wings of different aspect ratio.

Figure 9 from NACA Research Memorandum A55F06 (picture source)
Just so you don't think this can be overcome by clever design, below is a survey of the drag rise of supersonic aircraft, taken from a short course on fighter design by Ray Whitford. While engineers could lower the drag rise with increasing experience, the Sears-Haack body shows that there is a theoretical lower limit that cannot be avoided.

Supersonic drag rise survey by Ray Whitford. Note that here the absolute increase in overall drag is plotted. This graph includes all effects like trim drag which contribute to the supersonic drag rise.
However, this can be overcome by inserting a short dive which helps to gain enough kinetic energy to overcome the drag hump shortly past Mach 1. But you need to start with enough excess thrust so any net thrust is left in the supersonic region.
By the way, the thrust of your ideal rocket will increase on the way up because the atmospheric pressure drops. This will result in an increase in the pressure difference between the thrust chamber and outside pressure.

How far beyond Mach 1 does the 'drag hump' extend?

@SF. Depends on the aircraft. The F-16 never recovers but stays at a high value until Mach 1.6. Unswept aircraft have a very big hump (a factor of 5 to the subsonic drag coefficient is entirely possible) but recover quickly once fully supersonic. But never anywhere to their subsonic level.


text:
Do modern airplane controls provide force feedback of external forces on the control surfaces?
As I was sitting inside the cockpit of an old AN-26 at the aviation museum, long grounded and no longer flightworthy, sudden gusts of wind outside would sometimes move the control surfaces and make the yokes move around all on their own, spooking visitors with creepy thuds and groans of the hull and acting all creepy on the dark windy day. It seems in the days of old, with mechanical controls, it wasn't just the pilot acting on the control surfaces through the rudder - when attacked by winds, they would feed the external forces back to the yoke through the same system - likely both providing the pilot with important knowledge, and causing trouble by wringing the yoke in his hands.
I wonder how that looks like nowadays in modern planes, where the control is electronic, hydraulic, and a feedback like this would need to be specifically, additionally implemented, as an extra complication of the system instead of being in its inherent nature. Well, is it? Do the yokes of, say, modern airliners push harder on the pilot's hands when entering a turbulence or such?

For fly-by-wire, see this related question. Note that this is not a duplicate as it does not answer the question for hydraulic controls.

This question is vague and the answer will depend upon the responder's opinion of what "modern" means.  Plenty of "modern" aircraft are still cable and pulley, so this question could also be considered too broad.

@RyanMortensen: The current answer is perfectly satisfactory to me - the effect neither dead, nor omnipresent. The fact it's being implemented in some aircraft where it would normally be completely absent - and deliberately (not just for cost savings!) left out in others is just the overview I was looking for.

On some. The Fokker 100 is one, where the aeroforces on the elevator are directly fed back to the pilot. There is a hydraulic actuator helping him, but contrary to the usual solution it is a reversible system where the hydraulics merely amplifies the direct force that the pilot exerts.
The F100 has manual reversion for elevators and ailerons, and it can still be flown reasonable comfortably without hydraulics. The 737 also has manual reversion, but the flight controls are quite heavy without hydraulics - it is about the largest high-subsonic aircraft that can still be hand-flown. Larger planes just have triple redundant hydraulic controls, some with q-feel which artificially replicates the higher air stiffness felt at higher airspeeds. No romantic gusty creaking for those aircraft in a museum.

The Fokker 100 system is called a boosted system. And force feedback is one of the most important sources of information to the pilot. Designing the control system of large aircraft for manual control is a lost art. Sigh.

In Airbus' aircrafts this kind of feedback does not exist yet. The mechanism of the joystick does not take into account external information and the force and torque to be applied is always the same.
However, they are making a big effort to design new joysticks with a haptic feedback. The main idea is to include different string with a variable stiffness in order to modify the torque and force to be applied on the stick. With this system the pilot would be more aware "feeling" the real situation of the aircraft and could prevent some unsafe situations.

Do older Airbusses with yokes rather than joysticks (A300/A310) use force feedback?

Actually since the A320-style side-stick commands pitch and roll rates, the side-stick force mostly corresponds to the force on the control surfaces, because the rates do. It is position correspondence that is lost.


text:
How does ATC radar display differ from military radar?
Could you summarize what sort of data/objects are present in one type of display but not another? (not asking about construction differences, just the user-facing end, the display/interface.)
A somewhat related question "What data is displayed on civilian ATC radar screens?" shows example ATC displays. What of these would military radars miss (like obviously unknown data about enemy aircraft) and what extra would they present? Also - what sort of objects would appear on one but not another (in particular: non-aircraft; I heard for example flocks of birds sometimes appear on the radar, but never what kind of radar.)

This link
has numerous depictions of Raytheon made STARS (Standard Terminal Automation Replacement System)
Wikipedia says

The system is currently being used at a number of TRACON sites throughout the US2 and a number of military RAPCON sites that control military air traffic.[3]
The STARS is part of the FAA's TAMR project to replace aging hardware and software at TRACONS. It is also called TAMR Phase I.[4] The larger TRACONs still use Common ARTs capable equipment but the FAA announced in Spring 2011 that TAMR Segment 3 Phase 1 will consist of replacing these 11 larger TRACONS with STARS by 2016. Simultaneously, all Terminal areas in the United States will phase out all CARTS and ARTS systems and switch to STARS. The smaller sites will transitions to the STARS ELITE (Enhanced Local Integrated Tower Equipment) version of software and hardware, which is similar to TAMR, but with minimum redundancy. The FAA plans to complete this process by 2020.

So it would seem that in many locations there would be no difference.

Raytheon's DoD STARS, also known as DoD Advanced Automated Systems (DAAS), provides a state-of-the-art air traffic control system for managing terminal area airspace for the US military.
DoD STARS includes 191 RAPCON facilities with 59 associated towers and 21 stand-alone towers.
STARS receives radar data and flight plan information and presents the information to air traffic controllers on high resolution, 20" x 20" color displays allowing the controller to monitor, control, and accept hand-off of air traffic.
The Terminal Controller Workstations (TCW) provide position-by-position in-place replacement of failing existing system displays. They provide up to 14 adaptable data block types and 16 adaptable list types to accommodate current and future display requirements. The TCW also provides controllers with preference sets that allow for ease in shift changes and re-sectorization.
The color displays are specially developed for air traffic control and are capable of displaying six distinct levels of weather data (identified by different colors) simultaneously with air traffic, allowing controllers to direct aircraft around bad weather.
The color displays are specially developed for air traffic control and are capable of displaying six distinct levels of weather data (identified by different colors) simultaneously with air traffic, allowing controllers to direct aircraft around bad weather.
STARS is capable of tracking up to 1350 airborne aircraft simultaneously within a terminal area. The system interfaces with multiple radars (up to 16 short and long range), 128 controller positions, 20 remote towers, and a 400 by 400 mile area of coverage.
STARS has two separate, fully redundant automation systems running in parallel providing an instantaneous back-up service to controllers. The Full Service Level (FSL) is based on Raytheon's AutoTrac air traffic management system and the Emergency Service Level (ESL) is based on Raytheon's TracView automation product.


text:
What's the benefit of this long flight (apart from marketing)?
The post Do promotional flight routes have a historical precedent? asks about previous occurrences.
I'm guessing this flight (shown below) was a test flight, as well as being a marketing attempt.
For a test flight, what would the benefit be?

Related question: Flight path for promotion purposes - does this have a historical precedent?

@GregHewgill Bother - I did search the stack for n7874 and no matches were found.

I'm not saying you did anything wrong, I just happened to see both questions.

marketing came second, the first benefit is the flight testing.

From the this link:

At 15:38 local time N7874, the fourth 787 built, departed Boeing Field in Seattle for an 18-hour, 22 state test flight. The crew spent the overnight hours above the United States performing ETOPS testing on the new Rolls Royce Trent 1000 TEN engine, which will power the 787-10.

Also this video from the above flightradar blog link.
The 18-hour flight was to test ETOPS, and there's no harm in some marketing as well. Also since the flight will be coming back to Seattle, it makes sense to keep the flight above the US, and near airports. Compared to venturing out in the ocean on a test.


text:
In a turbofan what holds the spinning axis?
When the turbofan is spinning inside the housing, what holds the axis? All I know is that the axis holds the fan blades.

Stationary frames
The supporting structures of a turbofan are two stationary frames with their characteristic struts, located at the ends of the engine core: One is behind the OGV stator of the fan module, the other at the aft end of the turbine module:

CFM56 with the two frames visible (source)
Stationary frames to spinning shafts
In the case of an engine with two spools, the frames support the longest shaft (N1, low pressure), which in turn supports the coaxial N2 shaft (high pressure).

Shafts are centered within frames by bearings (drawn in magenta in the next picture).
Shaft to rotors
Disks of rotors on a spool are bolted together to form a continuous structure fixed to the related shaft. The structure is used as the inner wall for the related air duct:

Frames, shafts, bearings. Adapted from CFM56-7B Familiarization Manual
A view of the N1 shaft and the two frames, before adding the bearings and their oil sumps. The fan shaft (darker / trumpet-shaped) is fixed to the N1 shaft using grooves and can be considered part of the N1 shaft:

LP shaft, fan frame and turbine frame of a CFM56-7B
The bearings and the oil sumps are added to center the N1 shaft (blue tubes are used to pressurize air-oil seals of the fan sump). The clevises used to hang the engine under the pylon strut are (barely) visible on the top of the rear frame (see this question for details):

LP shaft, fan frame, turbine frame, bearings and sumps of a CFM56-7B
Another CG view which additional elements:

The fan disk bolted on the N1 shaft (actually fan shaft).

The fan cone and spinner bolted on the fan disk.

The two extremities of the N2 shaft coaxial to the N1 shaft.

Shafts of the CFM56-7B
The HP/N2 shaft (shown in red above) is centered on the LP/N1 shaft by bearings. All rotors of the HP spool form a single structure bolted to the HP shaft (shown in light blue below):

HP rotors of CFM56-7B. Adapted from CFM56-7B Familiarization Manual
Frames to core stators
Similarly, outer shrouds of stators and intermediate rings of a spool are bolted together to form continuous structures.
Four stationary structures are built:

The LP compressor (booster) stators assembly is fixed to the mid-box of the fan frame (on the fan side).
The HP compressor stators assembly is fixed to the mid-box of the fan frame (on the other side).
The combustion chamber is fixed to the HP compressor assembly.
The turbine stators assembly is fixed to the combustion chamber on one end and to the rear frame on the other end.

Overall CG view with some components highlighted:

CFM56-7B cutaway. Original CG model by vhfine3d, at DeviantArt
From frames to wing strut
The engine is supported by a pylon, which is supported by the wing. There are only two mount points connected to the pylon, located on the fan and rear frames. Thrust is usually transferred to the wing using the forward mount point.
If it is not practical to transmit the thrust from the forward mount, e.g. because space is lacking to build a strong mount, then thrust links are used to transfer thrust to the aft mount which transmits it to the pylon:

Sources: left, right
Other components
There are a few other components to support.

The air inlet is fixed on the fan case.

The fan case (which role is to create a duct for the fan and contain any broken fan blade piece) is fixed on the fan frame.

The nacelle, that is the fan cowls and the thrust reversers ducts, including the reversers, is fixed directly to the pylon. The nacelle is not really part of the engine.

The exhaust nozzle and cone are fixed to the turbine frame.

To sum up
Engine component are kept together using two frames.

Rotors are fixed to shafts, which are maintained within the frames by bearings.
Stationary components (fan case, stators, combustion chamber, etc) are bolted together and fixed the directly to the frames.

Nicely done! With some continued maintenance, we can probably start pointing most new jet engine questions at this answer and be done with it!  It's not that complete, but it's close!

Struts. Lots of them, and with an aerodynamic shape so they don't cause too much drag. Some of those struts double as stator vanes and reduce the swirl of the internal flow. By doing this they add a little thrust. Others house the driveshafts by which rotational energy is taken from the shaft to spin pumps and generators.
Maybe it helps to watch this YouTube video. I pasted a screen shot from it below:

Jet engine cutaway. The bright orange shows where the structure has been cut. In the grey gaps you see plenty of vanes and struts: They hold the shaft bearings in place.
On multi-spool engines the outer spool is held in place by the struts and has internal bearings to keep the inner spool in place. Since the outer, high-pressure spool is shorter, the inner, low-pressure spool emerges from the outer spool at both ends.
This drawing from a patent shows moveable stator vanes. They are arranged like spokes on a wheel. Each disk of stators is placed between two rotor disks.

Stator vanes (26 and 28) held moveably in place by bolts (32 and 44) and interspersed with rotor vanes (24). A lever (38) on each vane is moved by an actuator ring (36) and adjusts the incidence of the stator vanes.

The movable stators don't seem to be connected (via bearings) to the shafts at all, so they don't hold anything. This doesn't answer the question.

@Scrontch Why didn't you read the answer? Of course don't the movable stators hold the shaft - how would you think they do?

I did read the answer, but did you read the question? I was just trying to be helpful by explaining the reason for my downvote. Of course i see that the stators don't hold the shaft. That's exactly why i'm complaining that this isn't a good answer. I don't see why you even mention them at all, let alone making them half the space of your answer. Just adds confusion for nothing imho.

I'm assuming you're asking about the shaft support. In that case, the engine shaft is supported on the engine casing using bearings (the type of which may vary).

I Believe the 1st picture don't show the turbine frame, but the fan frame (By the way, i'm not sure if Turbine Frame can have use of the VBV System), after some research I believe that CF6 and CFM56 have the exact same model of Fan Frame, probably they shares some other parts too... And maybe some others GE motors get shared others same parts

Antoine, corrected! Thanks.


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Is it possible to build a turbojet engine with aluminum cans? (Soft drink cans)
I'm interested in building a working model of a turbojet engine, but I don't have a lot of supplies. So I'm wondering if it's possible to use aluminum cans for construction, for every body part. While we're on this topic, would the heat melt the aluminum?

See What material is used to make the hot sections of jet engines?. A model will still have high-temperature and high-pressure areas.

There was a junkyard wars episode where a team made a jet engine out of scrap metal, and it worked long enough to win the competition. Pretty epic and worth a look. I don't think aluminum soft drink cans are up to the task but there's other scrap material which could be used.

What materials do you recommend?

How about a pulse jet? They're supposedly much simpler to build.
http://www.pulsejetengines.com/how-to-make-a-pulse-jet/

@XTImpossible materials should be something with a high melting point. Steel might work for a short period... also suggestion by ROIMaison is a very good one...

Visit a junkyard and find an old turbocharger?

@ROIMaison Holy smokes, not a single disclaimer on that whole page! Hope the plans have the address of their attorney on them.

What do you mean?

Melt point of aluminum is 660.3 °C

How are you going to make the axis just from cans?

Cut the top and bottom of the can and roll the side up to form a stick?

Although i'm generally talking about a small engine the size of a water bottle. I checked the article, I prefer to build a turbojet engine.

Maybe a jet engine isn't something that should be built on a whim when short on supplies? Why can't you get actual materials and do it properly?

Because it's more challenging this way.

No, you cannot make an entire model jet engine out of aluminium.
Most fuels burn at 1000+C and aluminium melts at around 660C: Even if you keep the burning contained in airflow (thus away from the walls) the heat transfer will still damage the aluminium.
You could make the majority of the engine from it, though: Just as long as you find something more suitable for the 'hot' parts.

A quick Google search turned up that you can burn an Ethanol-water mixture at 700K (427 degrees C). And you might be able to wrap the fuel supply lines around the combustion chamber to preheat the fuel and simultaneously cool the chamber. Still, that first disk in the turbine will be running hot.


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How many bird strikes are there per year? Any world-wide statistics?
Does anyone know where I could find where the highest rates of bird strikes are?
Along the same lines, other wildlife, such as moose or bears on runways?

Have you checked http://wildlife.faa.gov/ ?

In collating any bird strike data, one must recognise that there is a tendency towards under reporting, especially when such strikes did not cause any damage. That said this report provides data for the UK & Canada  till 2009 https://easa.europa.eu/system/files/dfu/Final%20report%20Bird%20Strike%20Study.pdf

IATA reports provide the % of in-flight accidents, and the ratio of bird strikes among them. Incidents are not included though, and this is only for commercial flights.

For airliners , GA or both?

This graphic shows trends on birdstrikes to civil aircraft

This graph does not have sources.

Sorry, FAA and USDA publication. January 2019.


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The “Goodyear Airship Operations Manual”
I’m using ASA’s products to prepare for the AGI and IGI knowledge tests.  They often contain questions about airships and reference a document called the Goodyear Airship Operations Manual.  I’m already familiar with the K-Type Airship Pilot Manual which the FAA uses as a handbook (I presume) for LTA Airship training but I have not heard of the former document.  Does this thing exist and does someone have a copy of it as a PDF?  Thanks

Okay, even without studying for a new license, that sounds like a cool document to peruse!

The current Goodyear blimp is a Zeppelin NT semi-rigid airship. If you look for that instead of "Goodyear Blimp" you will find, among other things, the EASA type certificate EASA-AS-001, and it contains an excellent starting point for what you are trying to find.


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Why were the external stores pylons on the F/A-18E/F Super Hornet canted outward?

It is very visible in the picture above that the external stores and pylons to mount them on F-18 Super Hornets are canted outward.  I do not believe the legacy Hornets had their pylons mounted this way, so I wonder why it was changed on the Rhinos.  From an aerodynamics perspective, it seems to be counterproductive as this would cause a great deal more drag, then having the pylons and stores aligned with the relative wind. Is there some sort of structural advantage to it?  Does it improve performance when maneuvering or something similar?

Why were the external stores pylons on the F/A-18E/F Super Hornet canted outward?

For the same reason why the engines of a jetliner are slightly canted outward i.e. to follow the local airflow which is bent due to the presence of the wing and of the fuselage (source):

I suppose that this also helps in widening the safe-separation envelope of the weapons.

Possibly. However, there are some other complications here. That may well be beneficial on a high subsonic, high aspect ratio wing typical of jetliners, but may not necessarily be true for a low aspect ratio trapezoidal platform wing typically used on combat aircraft. There may also be other issues associated with i.e. missile exhaust ingestion by the engines or separation issues with freefall ordinance, potentially colliding with the fuselage or tailplane, etc.

The picture in the answer regards exactly a low aspect ratio swept wing like the one of the F-18. Unfortunately I couldn't find any better picture but the main idea is the same i.e. weapons are mainly pointing in the same direction as the airflow under the wing. Safe-separation has been also dealt with in my answer.


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Why do we have a grid-like layout of nav waypoints over the Gulf of Mexico
This is an unusual feature found, apparently only over the Gulf of Mexico south of the Continental United States. There appears to be an array of NAV waypoints, evenly laid out in an 18 x 20 nautical mile grid, both over US territorial waters and international waters. These waypoint do not be seem to be associated with airways, both VFR and IFR low and high altitude routes.  I have wonder if this is a military arrangement, for aircraft operating with the United States, military, and Allied nations, such as Mexico to conduct flight operations and military exercises there (a lot of these waypoints lie in the warning areas off the Gulf Coast).  Does anybody know what these waypoints are used for?

It appears these waypoints are part of the helicopter sectional for the Gulf of Mexico.  I Would imagine it allows pilots to create very specific flight plans to get to the oil rigs in the Gulf.  The sectional shows the waypoints that OP found as well as a numbered grid.
Make your Flight Plan at SkyVector.com

Looks like this may be an extension of the NRS, see page 3 here.

See AIM 10-1.4 The Gulf of Mexico Grid System: "Flight plan routes are routinely defined by just 4 segments: departure point (lat/long), first en route grid waypoint, last en route grid waypoint prior to approach procedure, and destination point (lat/long)."

@RalphJ  That was the first thing I thought of when I read this post.


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When buying an aircraft which has had multiple owners it is acceptable to get the logbooks from the previous owner only?
I know of a case where a small GA aircraft has had three owners since it was manufactured.  However the only airframe, power plant and propeller logbooks available for the airplane are from the most recent owner and no other logbooks are available from the two owners prior to the current owner.  The current owner's logs are good with the required annuals, engine overhauls, etc. documented and incorporates all FAA and manufacturer ADs to date.  Does this decrease the value of the aircraft?  My first thoughts are that yes, it will, as I have no idea as to what happened to the aircraft prior to the date the current owner obtained it, but I'm not totally sure.

It does decrease the value but may also be a good deal for a buyer going in for a long term purchase or a flight school leaseback machine.  I would not be opposed to buying a common GA plane with missing logs provided the plane "checked out" during the prebuy.  I'd pay nothing near VRef value, however.
Humid climate history, as much as could be determined, may be a non-starter.  Usage of <50hrs/yr over last 3-5 yrs would likely be a non-starter.  Obvious or poorly repaired damage is a guaranteed no-go.  An in-flight speed check would be mandatory as would an oil analysis on well-used oil (>10hrs).  Little rubber parts like gaskets and hoses can be good "attention to deal" indicators.  If the owner took care of little details the big stuff is probably fine too (still, verify everything!).
Nothing wrong with missing logs as long as the price is right and a good inspection is performed.
Edit:
What is the subject aircraft?
The value impact will depend a great deal on how far back the logs go.  If they only go back a few years on a ~40 year old plane then maybe a ~50% cut is in the ballpark.  If they go back 10yrs on the same plane then maybe ~30%.  Personally, I'd make a absurdly low offer.  The owner knows (or will very quickly learn) that he's playing to a tiny, tiny segment of the buying market.  Your borderline insulting cash offer may net you a servicable plane that will give you years of faithful service.
In my experience, a common GA plane with no verifiable damage history, unremarkable avionics, and decent (not "great") P&I will sell for the VRef price minus 10%, give or take a few bucks.
As they say, there is only one reason a plane won't sell: price is wrong.
As far as piecing together the history goes, check the NTSB database and google the N number.  People love taking pictures of damaged planes!  The 337 CD from the feds is a given...gotta do that.  Use the 337s to track down the shops.  If they're still in business give them a call and see if thay have any additional records or information on the plane.  They probably won't but it's worth a shot.  The 337s are also an indication of where the plane may have spent time.  A bunch of 337s from AZ would give me warm-and-fuzzies.  Not so much with a bunch of south FL records.

It will most definitely decrease the value of the airplane. How much is not easily answered. If the current owner has owned the plane for a long time and all the ADs are complied with then probably not much of a decrease in value. If they have replaced the engine and prop—even better. However, if the current owner has had it for a short while, then maybe he’s selling it for a reason related to the missing logs.
Maybe there was a prop strike or gear-up with a previous owner and they didn’t do the repairs properly. Or maybe there is a lien on the plane from when the last owner didn’t pay the shop. Or it could be that the plane was perfectly maintained but an ex-spouse kept the books in a nasty divorce. That’s why it is difficult to say how much the missing books decrease the value.
You can get the records for the airplane from the FAA in Oklahoma City to see if there was major damage properly repaired. The A&P is supposed to file 337s for major repairs and modifications.
In any case, you’ll need to have an A&P who is familiar with the type do a careful pre-buy.
Whether you should buy a plane with missing logs partly depends on how long you intend to keep it. Just like you are concerned about missing logs, the next owner will be too. If you plan to keep it for a long time, then you probably shouldn’t be too concerned about the difficulty of selling it later. If it’s a transition airplane for you, then know that it will be more difficult to sell.


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Does holding an MEI entitle you to instruct in ASEL/ASES as well?
Suppose someone earned a commercial pilot certificate in both ASEL and AMEL as well as only a  Multi Engine Instructor license.  Contingent upon both the MEI being valid and a valid Class II or better medical certificate, does the MEI allow this person to exercise the privilege of flight instruction in a single engine aircraft as well?

No. See FAR 61.195 (b):

Aircraft Ratings. A flight instructor may not conduct flight training in any aircraft for which the flight instructor does not hold:
(1) A pilot certificate and flight instructor certificate with the applicable category and class rating...

Without an Airplane - Single Engine rating on the flight instructor certificate, an instructor may not conduct flight training in a single-engine airplane.
As an aside, a CFI/I/MEI needs only a third class medical. See FAR 61.23(a)(3)(iv):

(3) [A person] must hold at least a third-class medical certificate— ...
(iv) When exercising the privileges of a flight instructor certificate and acting as the pilot in command;
(v) When exercising the privileges of a flight certificate and serving as a required pilot flight crewmember

In addition, note that these requirements are for an instructor also acting as a PIC or required pilot. When the student is the only required crewmember for a given flight, the instructor is not required to hold a medical certificate at all.

A CFI does not need a medical at all as long as the pilot in training is capable of acting as pilot in command and the flight doesn't require two pilots.  Many flight instructors teach commercial students that way.  Reread 61.23(a)(3)(iv) above and it agrees with my statement.

@wbeard52 if the pilot in training is capable of acting as PIC, then why does he need to be trained?

@kevin I assume you've taken a BFR? While not commercial, I just spent 10 hours with an instructor getting trained in a new aircraft (not a new type rating), and I was PIC the entire time...

@wbeard52 Good point! I was mostly responding to the Class 2 note in the question, but I've clarified the answer.

@kevin I actually find your comment disturbing.

I once spent 5 hours with a MEI getting checked out in a Seminole, brushing up on instrument flight, and completing an IPC. I was fully qualified in the FAA's eyes to operate as PIC for our flights. Apart from the IPC these 5 hours with an MEI were required by our insurance company. Incidentally this instructor got his MEI CFI first, and subsequently added ASEL CFI.


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Retreating Blade Stall, P-Factor, and Gyroscopic Precession
Most airplane pilots are aware of P-Factor in propeller driven aircraft and that it tends create a torque in phase with the lift imbalance over the blade disc, resulting in a yaw moment about the airplane's CG.  Curiously I'm surprised that it does not act to cause a pitch up moment as one would assume that to be the result of gyroscopic precession acting 90° out of phase with the application of the reaction forces.  This occurs that way with the retreating blade lift imbalance in helicopters leading up to a retreating blade stall if the airspeed is high enough through the rotor disc.  Retreating blade lift imbalance is a result of lower relative wind speeds passing over the retreating blade than the advancing blade.  But instead of resulting in a rolling moment towards the retreating side of the rotor disc, it results in a nose up pitching moment due to gyroscopic precession.  Which raises the question:  Why is gyroscopic precession present in a helicopter rotor due to lift asymmetry over the disc but absent in a propeller?  Gyroscopic precession causes phase shifts in resultant forces on propellers when the propeller disc is pitched or yawed in aerobatics or tailwheel aircraft operations, but P-Factor does not cause this as well?

Related, especially footnote 2: What is the influence of yaw on P-factor?
The answer to your specific question is that when you are talking about the effect of P-factor on a prop, you aren't talking about actually moving the prop disk in pitch or yaw.  You are applying a rudder force as needed to prevent that movement, so there's no gyroscopic effect to consider.
Whenever the prop disk is actually pitching or yawing-- regardless of whether the cause of the movement is P-factor or something else-- then you have to consider the gyroscopic effect, which makes the force act as if it is applied to the blade disk at a point 90 degrees further along in the direction of rotation.
In helicopters, the individual blades can be moving in a way that creates the same gyroscopic effect as pitching or rolling the entire blade disk, even when the blade disk as a whole is not pitching or rolling.  This is due to the way the blade is hinged.  This wouldn't happen with a rigid rotor disk.  So that's why-- at least with a hinged blade system-- the P-factor effect acts 90 degrees further along in the direction of rotation from the point where it is actually applied, even when the body of the helicopter isn't pitching or rolling, and even when the circular path traced by any given rotor blade tip is remaining constant over time.

Check this answer carefully; your mileage may vary--

I think that's about right. Precession will introduce a pitching moment that skews the resultant force only if you ALLOWED P factor to yaw the plane.

A “rigid” rotor allows blade flapping just like a hinged or teetering one does, just via flexing rather than hinges. But it does indeed transfer some torque directly, so the control forces act a bit less than 90° further, but still at least about 70°.


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Maneuvering Speed Discrepancy
In this page from the limitations section of the Extra Flugzeugbau 330SC, it states

I’m curious as to the discrepancy between maneuvering speeds Va between operation in the Normal Category and the different, and higher maneuvering speed Va when operated in the acrobatic category.
Maneuvering speed, as I understood it, what is the speed at which the airplane could be subject to a full deflection control input where, speeds below that would result in exceeding the critical angle of attack and stalling the airplane, and speeds above that, such an input could cause structural damage to the airframe, or sub components of that.
Now granted an Extra 330SC is built like a brick sh*thouse, allowing for a maximum load factor of +/- 10 Gs and some maneuvers can be entered at Vne without any problems.  That being said, I’m surprised to see the Va higher for acro flying that for normal category flight.  Anybody know what’s going on here? Is this a regulatory function, as opposed to a structural limitation?

It may be related to G limits between the 2 categories.  The NO seems to be based on the structural limits of the aircraft itself.  In other words, at a slower airspeed, an AoA increase must be greater to get the same Gs.

Every time I think I might be on to an answer it hits a dead end. This must be due to regulatory requirements regarding engineering safety factors for the two categories. I can't think of any physics reason why the structure would support more lift when operating in one category than the other. Is there a section of the POH that lists the operational assumptions for each category? (fuel load, accessories, and such) Is the fuel in the wings or fuselage? For small normal cat aircraft max wing lift is almost always used for Va, only in large planes where other loads like rudder may be limiting.


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Can you identify this antenna on a Cessna 172N?
Does anybody know what this antenna is used for?  I suspect a glideslope antenna but I am not totally sure.

Glideslope antennas are those U shaped stubby 3 prong fork things that look like heating elements for a water heater.  That's a 75 Mhz Marker Beacon antenna.  I have revised this post.  I thought it was just a VHF antenna, mainly because almost all Marker Beacon antennas you see out there these days are faired-in blade antennas, but the comment by @Gerry got me to do a bit more rummaging around, and found a couple examples of the same antenna as on that 172 and judging from the apparent length it's likely for Marker Beacon.  That antenna is probably quite old and I'm not even sure you can buy one today, as all the news ones seem to be blade style.

There are marker beacon antennas that look like this.  The length will tell you whether it's a comm antenna (about 23" or 60 cm) or a marker beacon antenna (about 39" or 1 m).


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Why does the C-2 left inboard stab not have a rudder?
As visible in the following picture
(wikimedia.org)
the left inboard stabilizer doesn't have a rudder but the right one does.
Wikipedia on the C-2 describes it as:

The C-2 has four vertical stabilizers, of which three are fitted with
  rudders. A single vertical stabilizer large enough for adequate
  directional control would have made the aircraft too tall to fit on an
  aircraft carrier hangar deck. The four-stabilizer configuration has
  the advantage of placing the outboard rudder surfaces directly in line
  with the propeller wash, providing effective yaw control down to low
  airspeeds, such as during takeoff and landing. The inner-left
  stabilizer lacks a rudder, and has been called the "executive tail",
  as it has nothing to do compared to the other three.

The rationale explains the primary design, but no explanation for the "executive" tail. Can anyone explain why the C-2 is designed this way?

What side is the critical engine on?

Port side engine (#1) is the critical engine.

If I have to guess, that one has a fixed angle to counter act the engines.

I'm guessing that this is to maintain directional control when the critical engine fails. With the critical engine running, not as much directional control is needed on that side. I can't find anything to support my theory though.

@ymb1 So the issue was 2 rudders are insufficient, but under what conditions? If it is limited to critical engine out, Ron Beyer may have a valid argument. Otherwise it may just be a cost/weight/efficiency decision, adding just the one additional rudder is the cheapest, lightest, and least complex solution. The critical engine may just drive which side it's on.

@RonBeyer  That may be a good argument for which side it's on, but why not have 4 rudders or even just enlarge the 2 outboard rudders? I'm trying to understand the design decision. And I've been in this business long enough to know that sometimes the 'solution' that seems strange in retrospect made perfect sense at the time. Designs evolve and it isn't always a linear process. I'm just curious how this came about.

@Gerry Since it is a carrier based aircraft, I assume that enlarging the other ones would take up too much space. By the way, the E-2 Hawkeye from which the C-2 was developed, has a similar tail setup. Here is a thread that discusses it. You should also note that both engines are "not handed" in that they spin in the same direction unlike many other twins that rotate in opposite directions.

I don't have an authoritative source for this, but I knew people who flew them, and the conventional wisdom was that even a fixed left inner was not required.  It was added simply for aesthetics and symmetry over an aerodynamic need.